Stratospheric variability and tropospheric ozone

Changes in the stratosphere-troposphere exchange (STE) of ozone over the last few decades have altered the tropospheric ozone abundance and are likely to continue doing so in the coming century as climate changes. Combining an updated linearized stratospheric ozone chemistry (Linoz v2) with parameterized polar stratospheric clouds (PSCs) chemistry, a 5-year (2001–2005) sequence of the European Centre for Medium-Range Weather Forecasts (ECMWF) meteorology data, and the University of California, Irvine (UCI) chemistry transport model (CTM), we examined variations in STE O3 flux and how it perturbs tropospheric O3. Our estimate for the current STE ozone flux is 290 Tg/a in the Northern Hemisphere (NH) and 225 Tg/a in the Southern Hemisphere (SH). The 2001–2005 interannual root-mean-square (RMS) variability is 25 Tg/a for the NH and 30 Tg/a for the SH. STE drives a seasonal peak-to-peak NH variability in tropospheric ozone of about 7–8 Dobson unit (DU). Of the interannual STE variance, 20% and 45% can be explained by the quasi-biennial oscillation (QBO) in the NH and SH, respectively. The CTM matches the observed QBO variations in total column ozone, and the STE O3 flux shows negative anomalies over the midlatitudes during the easterly phases of the QBO. When the observed column ozone depletion from 1979 to 2004 is modeled with Linoz v2, we predicted STE reductions of at most 10% in the NH, corresponding to a mean decrease of 1 ppb in tropospheric O3.